A. The Field of the Invention
The embodiments of the present invention relate to a Yagi antenna, and more particularly, the embodiments of the present invention relate to a portable Yagi antenna kit for being frequency/wavelength adjustable by virtue of being knockdownable.
B. The Description of the Prior Art
(1) General.
A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a directional antenna system1 consisting of an array of a dipole and additional closely coupled parasitic elements—usually a reflector and one or more directors. The dipole in the array is driven, and another element, 10% longer, operates as a reflector. Other shorter parasitic elements are typically added in front of the dipole as directors. This arrangement gives the antenna directionality that a single dipole lacks. 1 What is a Yagi-Uda antenna?—An explanation of the familiar Yagi-Uda antenna from a non-technical point of view. Includes information on wifi applications of Yagi Antennas.
Yagis are directional along the axis perpendicular to the dipole in the plane of the elements, from the reflector through the driven element and out via the director(s). If one holds out one's arms to form a dipole and has the reflector behind oneself, one would receive signals with maximum gain from in front of oneself.
Directional antennas, such as the Yagi-Uda, are also commonly referred to as beam antennas[2] or high-gain antennas—particularly for transmitting.
(2) Description.
Yagi-Uda antennas include one or more director elements, which, by virtue of their being arranged optimally at approximately a one-quarter-wavelength, mutual spacing and being progressively slightly shorter than a half wavelength, direct signals of increasingly higher frequencies onto the active dipole.
Thus, the complete antenna achieves a distinct response bandwidth determined by the length, diameter, and spacing of all the individual elements. But its overall gain is proportional to its length, rather than simply the number of elements.
All of the elements usually lie in the same plane, typically supported on a single boom or crossbar. The parasitic elements do not need to be coplanar, but can be distributed on both sides of the plane of symmetry.
The antenna gain is a function of the number of dipole elements and can be approximated—for the main lobe—as:GT=1.66*N where N is the number of elements—dipoles—in the Yagi-Uda antenna.
Developed Yagi-Uda antennas are designed to operate on multiple bands. The resulting design is made more complicated by the presence of a resonant parallel coil and capacitor combination—called a “trap” or LC—in the elements.
Traps are used in pairs on a multi-band antenna. The trap serves to isolate the outer portion of the element from the inner portion for the trap design frequency.
In practice, the higher frequency traps are located closest to the boom of the antenna. Typically, a tri-band beam will have 2 pairs of traps per element. For example, a typical tri-band Yagi-Uda beam covering the 10, 15, and 20 meter bands would have traps for the 10 and 15 meter bands.
The introduction of traps is not without cost—due to their nature, they reduce the overall bandwidth of the antenna and overall efficiency of the array on any given frequency, and radically affect its response in the desired direction.
(3) History.
The Yagi-Uda antenna was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Sendai, Japan, with the collaboration of Hidetsugu Yagi, also of Tohoku Imperial University. Yagi published the first English-language reference on the antenna in a 1928 survey article on short wave research in Japan and it came to be associated with his name. Yagi, however, always acknowledged Uda's principal contribution to the design, and the proper name for the antenna is, as above, the Yagi-Uda antenna—or array.
The Yagi was first widely used during World War II for airborne radar sets, because of its simplicity and directionality. Despite its being invented in Japan, many Japanese radar engineers were unaware of the design until very late in the war, due to internal fighting between the Army and Navy. The Japanese military authorities first became aware of this technology after the Battle of Singapore when they captured the notes of a British radar technician that mentioned “yagi antenna.” Japanese intelligence officers did not even recognize that Yagi was a Japanese name in this context. When questioned, the technician said it was an antenna named after a Japanese professor—this story is analogous to the story of American intelligence officers interrogating German rocket scientists and finding out that Robert Goddard was the real pioneer of rocket technology even though he was not well known in the US at that time.
Yagi-Uda antennas are widely used by amateur radio operators worldwide for communication on frequencies from shortwave, through VHF/UHF, and into microwave bands. Hams often homebrew this type of antenna, and have provided many technical papers and software to the engineering community.
Hidetsugu Yagi attempted wireless energy transfer in February 1926 with this antenna. Yagi and Uda published their first report on the wave projector directional antenna. Yagi managed to demonstrate a proof of concept, but the engineering problems proved to be more onerous than conventional systems.
(4) Standing Wave Ratio.
In telecommunications, standing wave ratio (“SWR”) is the ratio of the amplitude of a partial standing wave at an antinode—maximum—to the amplitude at an adjacent node—minimum—in an electrical transmission line.
The SWR is usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio. For example, the VSWR value 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times greater than the minimum standing wave value. It is also possible to define the SWR in terms of current, resulting in the ISWR, which has the same numerical value. The power standing wave ratio (PSWR) is defined as the square of the VSWR.
(5) Relationship to the Reflection Coefficient.
The voltage component of a standing wave in a uniform transmission line consists of the forward wave—with amplitude Vf—superimposed on the reflected wave—with amplitude Vr.
Reflections occur as a result of discontinuities, such as an imperfection in an otherwise uniform transmission line, or when a transmission line is terminated with other than its characteristic impedance. The reflection coefficient Γ is defined thus:Γ=Vr/Vf 
Γ is a complex number that describes both the magnitude and the phase shift of the reflection. The simplest cases, when the imaginary part of Γ is zero, are:                Γ=−−1: maximum negative reflection, when the line is short-circuited;        Γ=0: no reflection, when the line is perfectly matched; and        Γ=+1: maximum positive reflection, when the line is open-circuited.        
For the calculation of VSWR, only the magnitude of Γ, denoted by ρ, is of interest. Therefore, we define:ρ=|Γ|
At some points along the line the two waves interfere constructively, and the resulting amplitude Vmax is the sum of their amplitudes:Vmax=Vf+Vr=Vf+ρVf=Vf(1+ρ)
At other points, the waves interfere destructively, and the resulting amplitude Vmin is the difference between their amplitudes:Vmin=Vf−Vr=VfρVf=Vf(1−ρ)
The voltage standing wave ratio is then equal to:VSWR=Vmax/Vmin=(1+ρ)/(1−ρ)
As ρ, the magnitude of Γ, always falls in the range [0,1], the VSWR is always≧+1.
The SWR can also be defined as the ratio of the maximum amplitude of the electric field strength to its minimum amplitude, i.e. Emax/Emin.
(6) Further Analysis.
To understand the standing wave ratio in detail, we need to calculate the voltage—or, equivalently, the electrical field strength—at any point along the transmission line at any moment in time. We can begin with the forward wave, whose voltage as a function of time t and of distance x along the transmission line is:Vf(x,t)=A sin(ωt−kx)where A is the amplitude of the forward wave, ω is its angular frequency, and k is a constant—equal to ω divided by the speed of the wave. The voltage of the reflected wave is a similar function, but spatially reversed—the sign of x is inverted—and attenuated by the reflection coefficient ρ:Vf(x,t)=ρA sin(ωt+kx)
The total voltage Vt on the transmission line is given by the superposition principle, which is just a matter of adding the two waves:Vf(x,t)=A sin(ωt−kx)+ρA sin(ωt+kx)
Using standard trigonometric identities, this equation can be converted to the following form:Vt(x,t)+A✓[(4ρ cos2 kx)+(1−ρ)2] cos(ωt+φ)where: tan φ=[(1+ρ)(1−ρ)] cot(kx)
This form of the equation shows, if we ignore some of the details, that the maximum voltage over time Vmot at a distance x from the transmitter is the periodic function.Vmot=A✓[4ρ cos2 kx+(1−ρ)2]
This varies with x from a minimum of A(1−ρ) to a maximum of A(1+ρ), as we saw in the earlier, simplified discussion.
(7) Practical Implications of SWR.
The most common case for measuring and examining SWR is when installing and tuning transmitting antennas. When a transmitter is connected to an antenna by a feed line, the impedance of the antenna and feed line must match exactly for maximum energy transfer from the feed line to the antenna to be possible. The impedance of the antenna varies based on many factors including: the antenna's natural resonance at the frequency being transmitted, the antenna's height above the ground, and the size of the conductors used to construct the antenna.2 2 Hutchinson, Chuck, ed. (2000). The ARRL Handbook for Radio Amateurs 2001. Newington, Conn.: ARRL—The National Association for Amateur Radio. pp. g. 20.2. ISBN 0-87259-186-7.
When an antenna and feedline do not have matching impedances, some of the electrical energy cannot be transferred from the feedline to the antenna.3 Energy not transferred to the antenna is reflected back towards the transmitter.4 It is the interaction of these reflected waves with forward waves which causes standing wave patterns.5 Reflected power has two main implications in radio transmitters: Radio Frequency (RF) energy losses increase,6 and damage to the transmitter can occur.7 3 Hutchinson, Chuck, ed. (2000). The ARRL Handbook for Radio Amateurs 2001. Newington, Conn.: ARRL—The National Association for Amateur Radio. pp. 19.4-19.6. ISBN 0-87259-186-7.4 Ford, Steve (April 1997). “The SWR Obsession” (PDF). QST (Newington, Conn.: ARRL—The National Association for Amateur Radio. 78 (4): 70-72. Retrieved on Sep. 26, 2008.5 See footnote 3.6 Id.7 Hutchinson, Chuck, ed. (2000). The ARRL Handbook for Radio Amateurs 2001. Newington, Conn.: ARRL—The National Association for Amateur Radio. pp. g. 19.13. ISBN 0-87259-186-7.
Matching the impedance of the antenna to the impedance of the feed line is typically done using an antenna tuner. The tuner can be installed between the transmitter and the feed line, or between the feed line and the antenna. Both installation methods will allow the transmitter to operate at a low SWR, however, if the tuner is installed at the transmitter, the feed line between the tuner and the antenna will still operate with a high SWR, causing additional RF energy to be lost through the feedline.
Many amateur radio operators believe any impedance mismatch is a serious matter.8 This, however, is not the case. Assuming the mismatch is within the operating limits of the transmitter, the radio operator needs only be concerned with the power loss in the transmission line. Power loss will increase as the SWR increases, however, the increases are often less than radio amateurs assume. For example, a dipole antenna tuned to operate at 3.75 MHz—the center of the 80 meter amateur radio band—will exhibit an SWR of about 6:1 at the edges of the band. If, however, the antenna is fed with 250 feet of RG-8A coax, the loss due to standing waves is only 2.2 dB.9 Feed line loss typically increases with frequency, so VHF and above antennas must be matched closely to the feedline. The same 6:1 mismatch to 250 feet of RG-8A coax would incur 10.8 dB of loss at 146 MHz.10 8 See footnote 2.9 See footnote 3.10 Id.
Numerous innovations for antennas have been provided in the prior art, which will be described below in chronological order to show advancement in the art, and which are incorporated herein by reference thereto. Even though these innovations may be suitable for the specific individual purposes to which they address, however, they differ from the present invention in that they do not teach a portable Yagi antenna kit for being frequency/wavelength adjustable by virtue of being knockdownable.
(8) The U.S. Pat. No. 2,941,204 to Bailey.
The U.S. Pat. No. 2,941,204 issued to Bailey on Jun. 14, 1960 in U.S. class 343 and subclass 713 teaches an arrangement for supporting and for end-feeding an antenna, which includes an antenna element that is substantially a half wave length long, apparatus defining a ground plane, and cooperating and supporting apparatus for holding the element with its longitudinal axis generally perpendicular and with its lower end spaced from the plane. The coupling and supporting apparatus includes a resonant transformer coupled to the lower end of the antenna element and adapted to apply voltage thereto at an impedance substantially matched to that of the element. The outside surface of the coupling and supporting apparatus is conductive and has a length above the ground plane so that the surface is non-resonant at the frequency of operation whereby the radiation characteristic of the antenna is not adversely affected by the presence of the coupling and supporting apparatus.
(9) The U.S. Pat. No. 2,967,300 to Haughawout.
The U.S. Pat. No. 2,967,300 issued to Haughawout on Jan. 3, 1961 in U.S. class 343 and subclass 750 teaches a multiple band antenna including a plurality of coaxially related radiating elements of graduated length. Each of the radiating elements is shaped to radiate signals having different frequencies. At least one coaxial tuning sleeve is arranged to telescope between the radiating elements for isolating the signal frequencies radiated by one radiating element from the adjacent element.
(10) The U.S. Pat. No. 4,028,709 to Berkowitz et al.
The U.S. Pat. No. 4,028,709 issued to Berkowitz et al. on Jun. 7, 1977 in U.S. class 343 and subclass 819 teaches yagi antenna having a director element, a half-wave active dipole element, and a reflector element mounted on an antenna boom. All antenna elements are rods that are telescopically adjustable in length from a collapsible position to an operating length for a predetermined frequency of operation, and are removable from threaded mounting for storage. The director element and reflector element are slidably adjustable on the antenna boom for independent spacing with respect to the half-wave active dipole element. The antenna boom has two mast support holes, one for horizontal polarization and the other for vertical polarization. A ferrite core member surrounds a coaxial cable connecting the half-wave active dipole element to a coaxial connector, and provides balun action between the coaxial cable and a balanced antenna feed point.
(11) The U.S. Pat. No. 5,521,608 to Brandt et al.
The U.S. Pat. No. 5,521,608 issued to Brandt et al. on May 28, 1996 in U.S. class 343 and subclass 349 teaches a multi-band direction finding antenna including numerous antenna elements of coplanar location. The antenna elements associated with lower band frequencies are provided with chokes so that unchoked sections do not exceed one-quarter wavelength of the high-band highest frequency.
(12) The U.S. Pat. No. 5,995,061 to Schiller.
The U.S. Pat. No. 5,995,061 issued to Schiller on Nov. 30, 1999 in U.S. class 343 and subclass 815 teaches a no-loss, multi-band, adaptable Yagi style antenna employing a multi-element driven cell having a center element and one or more adjacent elements on each side of the center element. The adjacent elements of the driven cell are electrically shorter than the center element, thereby permitting the driven cell to be tuned to two or more frequency bands. The antenna is fed by a feedline connected to a common feed point at the center of the center element in the driven cell. Parasitic director elements are positioned in front of the driven cell and are tuned to the highest band of the driven cell. Parasitic reflector elements for one or more frequency bands are positioned behind the driven cell, with these elements tuned to actual operating frequencies of the antenna. A multi-band dipole antenna array covers three or more frequency bands, which includes a set of dipole elements having a center element and one or more adjacent elements and one or more adjacent elements on each side of the center element. The adjacent elements are electrically shorter than the center element and are of unequal lengths. The antenna is fed by a feedline connected to a common feedpoint at the center of the center element of the set of dipole elements. Parasitic director elements are positioned in front of the set of dipole elements, and parasitic reflector elements are positioned behind the set of dipole elements.
(13) The U.S. Pat. No. 6,154,180 to Padrick.
The U.S. Pat. No. 6,154,180 issued to Padrick on Nov. 28, 2000 in U.S. class 343 and subclass 722 teaches a parasitic antenna array (Yagi-Uda or loop type) for multiple frequency bands, which has its driven and parasitic elements interlaced on a single support boom. In a first aspect, series resonant circuits are located in one or more parasitic director elements in order to minimize the deleterious mutual coupling effect between directors of different frequency bands. In a second aspect, an inductance is placed across the feed point of the driven element of one or more non-selected frequency bands in order to minimize the bandwidth narrowing effect of closely-spaced driven elements and to provide a desired feed point impedance at the driven element of the selected frequency band. Although, the two aspects may be used without one another, they are advantageously employed together. In addition, the second aspect may be applied to closely-spaced driven elements that are not part of a parasitic array.
(14) The U.S. Pat. No. 6,677,914 to Mertel.
The U.S. Pat. No. 6,677,914 issued to Mertel on Jan. 13, 2004 in U.S. class 343 and subclass 815 teaches an antenna system with at least one tunable dipole element with a length adjustable conductive member disposed therein that enables the antenna to be used over a wide range of frequencies. The element is made of two longitudinally aligned, hollow support arms made of non-conductive material. Disposed longitudinally inside each element, is a length adjustable conductive member electrically connected at one end. In the preferred embodiment, each conductive member is stored on a spool that is selectively rotated to precisely extend the conductive member into the support arm. The support arms that may be fixed or adjustable in length are affixed at one end to a rigid housing. During use, the conductive members are adjusted in length to tune the element to a desired frequency. The antenna is especially advantageous when configured as a Yagi-style antenna that can be optimally tuned at a specific frequency for maximum gain, maximum front-to-back ratio, and to provide a desired feed point impedance at the driven element. The antenna can also function as a bi-directional antenna by adjusting the reflector element to function as a director. An electronic control system allows the length of the conductive members to be manually or automatically adjusted to a desired frequency.
It is apparent that numerous innovations for antennas have been provided in the prior art that are adapted to be used. Furthermore, even though these innovations may be suitable for the specific individual purposes to which they address, however, they would not be suitable for the purposes of the embodiments of the present invention as heretofore described, namely, a portable Yagi antenna kit for being frequency/wavelength adjustable by virtue of being knockdownable.